As it is well known, recirculation of part of the exhaust gas to the engine intake in order to reduce NOx emissions has become a common process. In fact, the exhaust gases are used for that purpose because they are oxygen-poor gases resulting from the combustion of the previous engine running cycles. Thus, introduction of such exhaust gases into the cylinders in the intake phase has the effect of decreasing the amount of fresh air that can mix with the fuel. This allows to lower the combustion temperature of the fuel mixture and consequently to reduce the production of NOx resulting from this combustion.
Generally, this exhaust gas recirculation is carried out by means of a circuit referred to as EGR (Exhaust Gas Recirculation) circuit, comprising a line connecting the exhaust manifold of the engine to the intake manifold thereof. This line is provided with a valve referred to as EGR valve, which allows to control the flow of gas circulating therein between a fully closed position and a fully open position of the valve.
A recirculated exhaust gas exchanger-cooler referred to as EGR cooler is advantageously provided, which has the effect of controlling the temperature of the exhaust gases before they are fed into the intake manifold. This cooler consists of tubes (or of gilled tubes) wherein a cooling fluid circulates and which are scavenged by the exhaust gases so as to exchange the calories of these gases with the cooling fluid.
The engine can also be a supercharged air type engine comprising a supercharging device such as a turbosupercharger, which compresses the ambient air it receives and sends it, in compressed state, to the engine intake. This turbosupercharger, usually arranged in the exhaust line, comprises a turbine that is driven in rotation by the exhaust gases from the engine that circulate in this exhaust line. The turbine is linked in rotation with a supercharger so that the rotation thereof compresses the ambient air before it is sent to the engine intake.
This supercharging device also comprises an exhaust gas discharge circuit allowing to control the amount of exhaust gas flowing through the turbine. A bypass line starting on the exhaust line upstream from the turbine and ending downstream from the turbine on this exhaust line is therefore provided. A discharge valve more commonly referred to as waste gate is arranged upstream from the turbine at the intersection of the bypass line and of the exhaust line. The waste gate can take a plurality of intermediate positions between a fully closed position where it closes the bypass line so that all of the exhaust gases flows through the turbine and a fully open position where it frees the inlet of the bypass line. This waste gate thus allows to control the amount of exhaust gases at the turbine inlet and, consequently, the pressure of the air leaving the supercharger.
These devices, although they give satisfactory results, however involve a certain number of drawbacks.
In fact, the large-size cold surfaces of a large part of the components of an engine such as those of the exchanger-cooler of the EGR circuit in particular are in contact with the hot exhaust gases containing unburned hydrocarbons, sulfur oxides, nitrogen oxides and soot particles. Soot particles can hang on to these surfaces whereas sulfur and nitrogen containing products can form corrosive acids as they condense on these surfaces, thus resulting in fouling and/or corrosion possibilities for the components in contact with the exhaust gases.
Thus, fouling of the outer surfaces of the tubes or gilled tubes of the exchanger-cooler can reduce its exchange capacity with the cooling fluid, thus causing degradation of the performances thereof. Furthermore, corrosion of the constituent parts of this exchanger-cooler by the acids reduces the life of the EGR circuit.
This fouling can also lead to a decrease in the functional capacities of some engine components such as EGR valves and waste gates, total sealing in the closed positions being then no longer provided, or such as injection nozzles some passages of which may be partly clogged by the soot particles.
Furthermore, the lines of the EGR circuit, of the discharge circuit and the exhaust line can undergo fouling due to the particles and/or corrosion due to the sulfur and nitrogen containing products.
The present invention aims to overcome the aforementioned drawbacks by means of components that are not subject to fouling through the exhaust gases.